Integrated semiconductor devices are typically constructed en masse on a wafer of silicon or gallium arsenide. Each device generally takes the form of an integrated circuit (IC) die, which is bonded to the die-mounting paddle of a leadframe. The wire attachment pads on the die are connected with their corresponding leads on the leadframe with aluminum or gold wire during a wire bonding process. The die and leadframe are then encapsulated in a plastic or ceramic package, which is then recognizable as an IC "chip". IC chips come in a variety of forms such as dynamic random access memory (DRAM) chips, static random access memory (SCRAM) chips, read only memory (ROM) chips, gate arrays, and so forth. The chips are interconnected in myriad combinations on printed circuit boards, along with the other types of discrete components such as resistors and capacitors, by any number of techniques, such as socketing and soldering.
For very high-temperature applications, components may be welded to traces on circuit boards. However, for most applications, components are soldered to circuit boards. The term solder refers to the soft metal alloy which is commonly used to join or fuse the surfaces of other metals or alloys having much higher melting points. Typically tin-lead alloys are utilized for electronics assembly purposes. There are two principal soldering techniques currently used for the attachment of components to circuit boards. The first is the soldered through-hole technique, whereby components are mounted on the top surface of a circuit board, with the leads of the components extending through metal-plated through-holes in the board which are of slightly larger diameter than the leads. The board is then subjected to a wave-soldering process. During the wave soldering process, solder is drawn by capillary action into the clearances between the leads and the through-hole walls. When the solder is allowed to solidify, the leads are securely soldered within the through-holes. The second principal soldering technique for attaching components to a circuit board is the surface-mount process.
The surface-mount soldering technique is utilized, for example, in the assembly of the Single Inline Memory Module (SIMM) depicted in a partially-assembled state in the top plan view of FIG. 1. The module, when complete, comprises 9 DRAM chips, D1 through D9, and 9 decoupling capacitors, C1 through C9, all of which are surface mounted to the printed circuit board 11. As a space-saving measure, each of the 9 decoupling capacitors is surface-mounted beneath one of the DRAM chips. Referring now to both FIGS. 1 and 2, each of the DRAM chips has 18 J-shaped leads which are soldered, at infrared-energy-generated temperatures of approximately 215 degrees Centigrade, to 18 chip-connector pads 13 on circuit board 11 beneath each chip. Each of the decoupling capacitors is also infrared soldered to corresponding pair of capacitor connector pads 15. The soldering of the DRAM chips and the capacitors is performed in a single heating operation. In the view of FIG. 1, the left-most DRAM chip has been removed, exposing one of the 9 sets of 18 chip-connector pads, as well as one of the 9 decoupling capacitors. In addition, the right-most DRAM chip and the decoupling capacitor beneath it have been removed, exposing a second set of chip-connector pads 13 and one of the 9 pairs of capacitor connector pads 15.
Prior to the attachment of the components to the board, the leads of the DRAM chips 21 and the capacitor terminals are tinned with 63-37 solder and the mounting pads on the printed circuit board are screen printed with a paste comprised of soldering flux and powdered 63-37 solder. The individual components are then positioned on the circuit board 11 so that, as nearly as practicable, the leads (the term "surface-mount leads" is used herein to refer both to surface-mount leads and terminals) of the components are located directly on top of their respective mounting pads. Once the components are thus positioned, the board-component assemblies are sent through an infrared oven and subjected to increasing temperatures similar to those recorded on the temperature vs. time graph of FIG. 3. At a temperature of approximately 183 degrees Centigrade, the solder plating (tinning) on the leads and the powdered solder on the pads melt. In order to assure that the solder on the leads and pads completely melts and fuses, the oven temperature is allowed to peak at 215 degrees Centigrade. The use of a temperatures greater than 215 degrees does not improve solder reflow for the particular solder alloy used and may decrease the reliability of the board's electronic components. When the assembly is allowed to cool, the leads or terminals of the components are firmly soldered to the connector pads. Proper alignment of the components is critical, since optimum connection of the components to the board will occur only if each lead or terminal is centered on its corresponding pad; poor alignment of components may result in an open circuit or a high-resistance connection.